1. Field of the Invention
This invention relates to dimmer circuits for high intensity, gaseous discharge (HID) lamps and more particularly to such a dimmer that provides dimming current to the lamp through at least partial ballast reactive bypass.
2. Description of the Prior Art
U.S. Pat. No. 3,816,794, Snyder, describes a circuit employing a two-part reactive ballast connected in series with a high intensity, gaseous discharge lamp. One of the two elements of the ballast is connected across the main terminals of a triac operating as a gated bypass means. When the triac conducts a current path is established through the triac, at least partially bypassing the reactive element. The duration of conduction determines the total amount of current through the ballast, and hence through the lamp, thereby providing a means for establishing the brightness of the lamp.
In the circuit described in U.S. Pat. No. 3,816,794, low gate source or drive voltage to the gate of the gates bypass triac is derived from a potentiometer, an isolating transformer circuit, a second triac and a Zener diode network, together with other components. The gated bypass triac is fired from a gate source voltage in phase with line voltage, the amplitude being controlled by a gate-signal control device including a Zener diode to properly time the turning on of the triac in relation to lamp current. The Zener diode also prevents the triac from remaining conductive past a time when there might be opposite polarity ballast-element voltage and lamp current, which would cause flicker of the lamp. Connection to multiple lamp circuits and to three phase systems was cumbersome, and isolation of the triggering of the gated bypass triac and the power for the circuit was incomplete.
U.S. Pat. No. 3,894,265 discloses a circuit that provides a control network for the gated bypass network including the programmable unijunction transistor. Ready connection to single power and three phase power systems is achieved, but the gating of the bypass triac is still not independent of the ac distribution voltage.
It is therefore a feature of the present invention to provide an improved dimmer circuit having a bypass triac or other gated means for at least partially bypassing a reactive ballast element connected to an HID lamp, the voltage source for driving the gated bypass means and the activation of such source being isolated.
It is another feature of the present invention to provide an improved dimmer circuit having a gated bypass means for at least partially bypassing a reactive ballast element connected to an HID lamp, the voltage source for driving the gated bypass means and the activation of such source being isolated by way of an optically isolated driver means, such as an optocoupler.
It is still another feature of the present invention to provide an improved dimmer circuit having a gated bypass means for at least partially bypassing a reactive ballast element connected to an HID lamp, the voltage source for driving the gated bypass means being through the operation of the driver portion of an optocoupler connected to the gated bypass means, a pulse activated receiver portion activating the driver portion.
It is yet another feature of the present invention to provide an improved dimmer circuit having a gated bypass means, a pulse activated receiver portion activating the driver portion, the pulse actuation being derived from high frequency bursts superimposed on the ac distribution line to the circuit.